1. Field of the Invention
The present invention relates to a process for the production of cyclic isocyanate addition products containing uretdione groups, to the resulting isocyanate addition products and to their use for the production of heat-curable coating compositions.
2. Description of the Prior Art
One-component polyurethane coating compositions derived from blocked polyisocyanates and polyols are an important class of materials for applications such as automotive OEM coatings. These one-component compositions are used because of the difficulties encountered when using two-component coating compositions in industrial applications. In the two-component compositions the polyisocyanates are not blocked, which results in several disadvantages. The two-component compositions must be accurately mixed or the properties of the resulting coatings can be substantially affected. In addition, after the components are mixed they have a limited pot life since the components continue to react until an unusable solid is obtained.
These disadvantages are not present in one-component coating compositions containing polyols and polyisocyanates blocked with reversible, monofunctional blocking agents for isocyanate groups. However, there are also disadvantages with one-component coating compositions, which are primarily caused by the volatilization of the blocking agent. The release of the blocking agent can cause blistering and yellowing in thick films and oven fouling. In addition, the blocking agents are considered to be volatile organic compounds (VOCs) in the same manner as organic solvents. Therefore, certain coating compositions may not satisfy environmental regulations solely due to the presence of blocking agents.
It is an object of the present invention to overcome the known disadvantages of one-component coating compositions caused by the presence of blocking agents without affecting the advantages of these coating compositions when compared to two-component coating compositions. This object can be achieved with the one-component compositions according to the present invention which contain special cyclic isocyanate addition products. The isocyanate addition products do not require blocking agents and, thus, do not release blocking agents during cure. The one-component compositions are stable liquid systems at room temperature and yet can be converted to isocyanate addition products under relatively mild thermal conditions without the release of volatile blocking agents.